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					           Hubs, Bridges, and Switches

r Used for extending LANs in terms of geographical
  coverage, number of nodes, administration
  capabilities, etc.
r Differ in regards to:
    m   collision domain isolation
    m   layer at which they operate

r   Different than routers
    m   plug and play
    m   don’t provide optimal routing of IP packets
r   Layer 2 and below components

                                      5: Link Layer and Local Area Networks   5c-1

r Physical Layer devices: essentially repeaters
  operating at bit levels: repeat received bits on one
  interface to all other interfaces
r Hubs can be arranged in a hierarchy (or multi-tier
  design), with a backbone hub at its top

                               5: Link Layer and Local Area Networks   5c-2
                       Hubs (more)

r Each connected LAN is referred to as a LAN
r Hubs do not isolate collision domains: a node may
  collide with any node residing at any segment in
  the LAN

r   Hub Advantages:
    m   Simple, inexpensive device
    m   Multi-tier provides graceful degradation: portions of the
        LAN continue to operate if one of the hubs malfunction
    m   Extends maximum distance between node pairs (100m per

                                      5: Link Layer and Local Area Networks   5c-3
                        Hubs (more)

r   Hub Limitations:
    m   Single collision domain results in no increase in max
        throughput; the multi-tier throughput same as the the
        single segment throughput

    m   Individual LAN restrictions pose limits on the number of
        nodes in the same collision domain (thus, per Hub); and on
        the total allowed geographical coverage

    m   Cannot connect different Ethernet types (e.g., 10BaseT
        and 100baseT)

                                       5: Link Layer and Local Area Networks   5c-4

r Link Layer devices: they operate on Ethernet
  frames (i.e. layer 2 devices), examining the frame
  header and selectively forwarding a frame base on
  its destination
r Bridge isolates collision domains since it buffers
r When a frame is to be forwarded on a segment,
  the bridge uses CSMA/CD to access the segment
  and transmit

                              5: Link Layer and Local Area Networks   5c-5
                  Bridges (more)

r Bridge advantages:
   m Isolates collision domains resulting in higher
     total max throughput, and does not limit the
     number of nodes nor geographical coverage

   m   Can connect different types of Ethernet since
       it is a store and forward device

   m   Transparent: no need for any change to hosts
       LAN adapters

                               5: Link Layer and Local Area Networks   5c-6
Backbone Bridge

          5: Link Layer and Local Area Networks   5c-7
    Interconnection Without Backbone

r   Not recommended for two reasons:
    - Single point of failure at Computer Science hub
    - All traffic between EE and SE must path over CS segment

                                   5: Link Layer and Local Area Networks   5c-8
                     Bridge Filtering

r Bridges learn which hosts can be reached through
  which interfaces and maintain filtering tables
r A filtering table entry:
        (Node LAN Address, Bridge Interface, Time Stamp)
r   Filtering procedure:
    if destination is on LAN on which frame was received
        then drop the frame
        else { lookup filtering table
                if entry found for destination
                   then forward the frame on interface indicated;
                   else flood; /* forward on all but the interface on
                                which the frame arrived*/

                                         5: Link Layer and Local Area Networks   5c-9
                 Bridge Learning

r   When a frame is received, the bridge “learns”
    from the source address and updates its filtering
    table (Node LAN Address, Bridge Interface, Time

r   Stale entries in the Filtering Table are dropped
    (TTL can be 60 minutes)

                                5: Link Layer and Local Area Networks 5c-10
               Bridges Spanning Tree
r   For increased reliability, it is desirable to have redundant,
    alternate paths from a source to a destination
r   With multiple simultaneous paths however, cycles result on
    which bridges may multiply and forward a frame forever
r   Solution is organizing the set of bridges in a spanning tree
    by disabling a subset of the interfaces in the bridges:
    Bridges talk to each other!

                                       5: Link Layer and Local Area Networks 5c-11
          Bridges versus Routers

r Both are store-and-forward devices, but Routers
  are Network Layer devices (examine network layer
  headers) and Bridges are Link Layer devices
r Routers maintain routing tables and implement
  routing algorithms; bridges maintain filtering
  tables and implement filtering, learning and
  spanning tree algorithms

                             5: Link Layer and Local Area Networks 5c-12
            Routers versus Bridges

r   Bridges + and –

+ Bridge operation is simpler requiring less
   processing bandwidth (plug and play)

- Topologies are restricted with bridges: a spanning
  tree must be built to avoid cycles

- Bridges do not offer protection from broadcast
  storms (endless broadcasting by a host will be
  forwarded by a bridge)

                               5: Link Layer and Local Area Networks 5c-13
            Routers versus Bridges

r  Routers + and -
+ Arbitrary topologies can be supported, cycling is
   limited by TTL counters (and good routing prots)
+ Provide firewall protection against broadcast
- Require IP address configuration (not plug and
- Require higher processing bandwidth

r   Bridges do well in small (few hundred hosts) while
    routers are required in large networks (thousands
    of hosts)
                                5: Link Layer and Local Area Networks 5c-14
             Ethernet Switches

r A new device came in 1990s
r A switch is a device that incorporates bridge
  functions as well as point-to-point “dedicated
r A host attached to a switch via a dedicated point-
  to-point connection; will always sense the medium
  as idle; no collisions ever!
r Ethernet Switches provide a combinations of
  shared/dedicated, 10/100/1000 Mbps connections

                              5: Link Layer and Local Area Networks 5c-15
           Ethernet Switches (more)

r Some Ethernet switches support cut-through
  switching: frame forwarded immediately to destination
  without awaiting for assembly of the entire frame in
  the switch buffer; slight reduction in latency (rather
  than store-and-forward packet switching)
r The cut-through switching will differ from store-and-
  forward switching only when the output buffer is
r When output buffer is empty, there is no need to get
  the whole packet before sending it out in cut-through

r   Ethernet switches vary in size, with the largest ones
    incorporating a high bandwidth interconnection
                                   5: Link Layer and Local Area Networks 5c-16
Ethernet Switches (more)



              5: Link Layer and Local Area Networks 5c-17
          IEEE 802.11 Wireless LAN

r   Wireless LANs are becoming popular for mobile
    Internet access
r   Applications: nomadic Internet access, portable
    computing, ad hoc networking (multihopping)
r   IEEE 802.11 standards defines MAC protocol;
    unlicensed frequency spectrum bands: 900Mhz,
r   Basic Service Sets +
    Access Points ®
    Distribution System
r   Like a bridged LAN
    (flat MAC address)

                                5: Link Layer and Local Area Networks 5c-18
              Ad Hoc Networks

r IEEE 802.11 stations can dynamically form a group
  without AP
r Ad Hoc Network: no pre-existing infrastructure
r Applications: “laptop” meeting in conference room,
  car, airport; interconnection of “personal” devices
  (see; battelfield; pervasive
  computing (smart spaces)
  (Mobile Ad hoc Networks)
  working group

                               5: Link Layer and Local Area Networks 5c-19
        IEEE 802.11 MAC Protocol
CSMA Protocol:
- sense channel idle for DISF sec (Distributed Inter
   Frame Space)
     - transmit frame (no Collision Detection)
     - receiver returns ACK after SIFS (Short Inter
       Frame Space)
-if channel sensed busy
   then binary backoff

NAV: Network Allocation
 (min time of deferral)

                             5: Link Layer and Local Area Networks 5c-20
          Hidden Terminal Effect

r CSMA inefficient in presence of hidden terminals
r Hidden terminals: A and B cannot hear each other
  because of obstacles or signal attenuation; so,
  their packets collide at B
r Solution? CSMA/CA
r CA = Collision Avoidance

                             5: Link Layer and Local Area Networks 5c-21
Collision Avoidance: RTS-CTS exchange
• CTS “freezes” stations within range of receiver (but
possibly hidden from transmitter); this prevents collisions by
hidden station during data
• RTS and CTS are very short: collisions during data phase
are thus very unlikely (the end result is similar to Collision

•Note: IEEE 802.11
allows CSMA, CSMA/CA
 and “polling” from AP

CTS: clear to send
RTS: request to send

                                    5: Link Layer and Local Area Networks 5c-22
       Point to Point Protocol (PPP)
r Point to point, wired data link easier to manage
  than broadcast link: no Media Access Control
r Several Data Link Protocols: PPP, HDLC, SDLC,
  Alternating Bit protocol, etc
r PPP (Point to Point Protocol) is very popular: used
  in dial up connection between residential Host and
  ISP; on SONET/SDH connections, etc
r PPP is extremely simple (the simplest in the Data
  Link protocol family) and very streamlined

                              5: Link Layer and Local Area Networks 5c-23
                PPP Requirements

r   Packet framing: encapsulation of IP packets
r   bit transparency: must carry any bit pattern in the
    data field of the network protocol
r   error detection (no correction)
r   multiple network layer protocols
r   connection liveness (detection of link failure)
r   Network Layer Address negotiation: Hosts/nodes
    across the link must learn/configure each other’s
    network address (e.g. IP address)

                                5: Link Layer and Local Area Networks 5c-24
             Not Provided by PPP

r error correction/recovery
r flow control
r sequencing
r multipoint links (e.g., polling)

                                 5: Link Layer and Local Area Networks 5c-25
                PPP Data Frame

r Flag: delimiter (framing)
r Address: does nothing (only one option)
r Control: does nothing; in the future possible
  multiple control fields
r Protocol: upper layer to which frame must be
  delivered (e.g., PPP-LCP, IP, IPCP, etc)

                              5: Link Layer and Local Area Networks 5c-26
                  Byte Stuffing
r For “data transparency”, the data field must be
  allowed to include the pattern <01111110> ; ie, this
  must not be interpreted as a flag
r to alert the receiver, the transmitter “stuffs” an
  extra < 01111110> byte after each < 01111110> data
r the receiver discards each 01111110 followed by
  another 01111110, and continues data reception

                                 5: Link Layer and Local Area Networks 5c-27
          PPP Data Control Protocol

r PPP-LCP establishes/releases the PPP connection;
  negotiates options
r Starts in DEAD state
r Options: max frame length; authentication protocol
r Once PPP link established, IPCP (Control Protocol)
  moves in (on top of PPP) to configure IP network
  addresses etc.

                                5: Link Layer and Local Area Networks 5c-28

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